This invention relates to computer communications networks operating in accordance with a token ring access protocol and, more particularly, to a token ring concentrator for use in such networks.
Token ring networks are well defined and standardized networks by which computers connected to the network may communicate with each other. A token ring network includes a trunk which comprises a normally closed ring and includes a token ring concentrator having a number of input/output ports through which, by means of relay switching, computers may be inserted into the network. A standard governing token ring networks is the IEEE Standard 802.5, wherein each of the input/output ports is referred to as a trunk coupling unit. According to this standard, insertion of a computer into the ring is controlled by the computer itself, with the mechanism for effecting the insertion or bypass of the computer residing in its respective input/output port of the concentrator. Each computer is connected to a respective input/output port of the concentrator via a medium interface cable (MIC) and the computer exercises control of the insertion/bypass mechanism via the MIC using a phantom circuit technique. The phantom circuit impresses a DC voltage on the MIC. This DC voltage is transparent to the passage of computer-transmitted symbols, hence the name "phantom". The impressed voltage is used within the input/output port of the concentrator to effect the transfer of a switching action to cause the serial insertion of the computer in the ring. Cessation of the phantom drive causes a switching action which will bypass the computer and cause the computer to be put into a looped (wrapped) state. This loop may be used by the computer for off-line self-testing functions.
The IEEE Standard 802.5 illustrates relay contacts for effecting the switching action to bypass/insert the computer. Typically, a latching relay is used because the impressed phantom voltage, and the resulting phantom current, is not sufficient to continuously power a relay. Instead, a capacitor is usually provided to store energy, and at the appropriate time the capacitor is discharged through the relay coil to provide the burst of energy required to switch the relay.
In a system of the type described, a typical latching relay includes a set coil and a reset coil, with the set coil being energized to effect insertion of the computer into the ring and the reset coil being energized to effect bypass of the computer from the ring. It is often desired to provide a global reset capability which results in simultaneously bypassing all of the computers coupled to the input/output ports of a concentrator. This requires that all of the reset coils be energized simultaneously. It is therefore an object of the present invention to provide a global reset arrangement allowing for the simultaneous energization of all of the reset coils in a token ring concentrator.
If the reset coils are used by the insertion/bypass control mechanisms in the input/output ports as well as by the global reset arrangement, some means must be provided for isolating the reset coils from each other while still coupling all of the reset coils to the global reset arrangement. Accordingly, it is a further object of this invention to provide an arrangement which effects such isolation. If a port is reset while in use, this can create an undesirable condition for the computer connected to that port. It is therefore another object of this invention to provide an arrangement which prevents activation of the global reset function from affecting an input/output port that is active.